The present invention relates to the provision of an environmental seal around a splice in one or more optical fibre cables.
Optical fibre cables comprise a bundle of optical fibres surrounded by a protective jacket. Since such cables are manufactured in finite lengths, any installation will in general require separate lengths of cable to be spliced together. A problem arises at the splice, not only in forming fibre splices of low light attenuation, but also in replacing the protective jacket which has to be removed in the region of the splice to expose the fibres. It is with this second problem that the present invention is concerned.
Splicing an optical fiber is a difficult business, requiring use of a machine for proper alignment of pairs of fibres before a permanent splice is made between each pair. The machine will of necessity be some short distance away from the ends of the intact portions of the cables to be joined, i.e. some distance from the ends of each cable jacket. The jacket of each cable to be joined may be stripped back a distance of, say, 1.5 m thereby exposing 1.5 m of optical fibres. It can be seen, therefore, that there is a further problem, namely the accommodation of a significant length of optical fibres within the environmental housing which is to replace the removed portions of cable jacket. Optical fibres are easily broken by being bent around too sharp a radius, and the housing must therefore provide some way of organizing the fibres along an acceptable path.
There are some further considerations. It will generally be desirable that any chosen fibre within the housing be accessible, and therefore the fibres, which may number hundreds, should be organized within the housing in a regular fashion. Also, some means of identifying each fibre is desirable, and such means may have to be by way of its location within the housing since color coding of the fibre itself is difficult.
Various suggestions have been proposed for an optical fibre splice case which organizes the slack lengths of fibres and provides an environmentally sealed housing around them.
In European patent publication 0043570 there is proposed an optical fibre splice case through one side of which one cable enters and through an opposite side of which another cable enters. The ingoing cables are sealed to inlets of the case by shrinkable sleeves. Within the case is a series of plates, each with projections thereon around which are wound the spliced fibres, thereby storing the slack lengths in a path which does not result in damage to the fibres. Such a splice case is known as an in-line splice case since the cables enter the case from opposite ends, or a least from widely spaced positions. This may be contrasted with a butt cable splice where the cables are substantially mutually adjacent.
A similar series of plates for use in an in-line cable splice is disclosed in U.S. Pat. No. 4,266,853. Here the plates are stacked one on top of another and are hinged to a carrier. The hinging allows a chosen plate to be revealed for attention to a fibre splice it carries.
Optical fibre cables may be, for example buried, aerial, or suspended between poles. In each case in addition to providing environmental protection, some mechanical strength has to be provided, and particular attention has to be given to axial pull strength in the case of aerial installations. Many of the prior art designs provide for axial pull strength in the following way. The cables themselves may be provided with a steel wire core, around which the fibres are located. In addition to preventing excessive bending of the cable, the core can be used to transmit axial load through the splice case, thus eliminating axial load on the fibres themselves. The way this is done is to strip back the jacket of each cable in the usual way to expose sufficient lengths of fibres for the splicing process to be carried out. The metal cores are cut to leave a short length protruding beyond each remaining cable jacket. The metal core of one cable is then clamped to one end of the splice case and the core of the other cable is clamped to the opposite end of the splice case. Thus, any axial load is transmitted from one core in a line through the case to the other core. Such an arrangement is disclosed in European Publication No. 0077115.
Various other designs for optical fibre splice cases are disclosed in DE 3006131, JP-A-55-127508, JP-A-55-127507, JP-A-55-100504 and JP-A-55-133146.
Although most of the designs proposed in the prior art are likely to be satisfactory each has a disadvantage due to its basic cable geometry. The splice cases tend to be bulky, difficult to remove from a manhole or pole where they are located, and difficult to re-enter for repair or testing. We have discovered that these problems can be overcome by a simple modification to the arrangement of the cables and organizer plates.